The technology that enables us to "look" beneath the ground and locate hidden objects is described by a number of terms that can be difficult to compare without some explanation. This guide will help you to understand the differences in the technology and make an informed buying decision.
Losing things is a fact of life, especially small things like coins, rings, watches, bracelets and other jewelry items. Finding them can be fun, lots of fun. Over the last 250 years of American history, literally millions of such items have been dropped, or in some cases buried, and lie somewhere under the earth’s surface waiting to be discovered.
If you’re the type of individual that enjoys finding things, metal detecting might be a hobby that’s just right for you. The first challenge to finding bliss while unearthing buried treasure is selecting the right metal detector.
Like so many other devices, technology has improved the equipment used for this fascinating pastime and there are numerous features that make it much easier to locate and determine what the object might be before you dig. As with any category of electronic device, price is relative to features, flexibility and power. The further you move up the price scale, the more sophisticated the units, and therefore their effectiveness. While features such as discrimination circuitry and target identification will enhance the experience, you still have to learn how to use them and master the various adjustments to achieve maximum performance. Sound-only detectors have the ability to locate objects as effectively as the more sophisticated Target ID units; they just take a little more experience to operate.
How They Work
There are several different types of metal detector technology, but the one that is the most widely used is Very Low Frequency (VLF). VLF is also called induction balance, because it uses both a transmitter and receiver coil, which analyzes signals that are induced by the detector’s current oscillation.
Search coils are key components when it comes to finding buried metallic objects. They come in various sizes, and all will have actually two coils that work together. The transmitter coil is the larger outer coil loop. When activated, electricity is sent through the transmitter coil in alternating directions thousands of times per second, which generates a magnetic field. The number of times that the current alternates establishes the frequency of the unit. As the coil pulsates it changes the polarity of the magnetic field. The magnetic field is constantly being projected down into the ground and drawn out again. As the field comes into contact with conductive objects it causes them to generate weak magnetic fields of their own.
The receiver coil is the smaller inner coil. This coil is totally shielded from the transmitter coil. It serves to pick up and amplify the frequencies being reflected back from target objects. Once an object returns a signal to the receiver coil it is amplified and sent to the control box where it is analyzed as to content and depth and the results are displayed.
Since VLF is the most commonly used system, the first decision you’ll have to make will be how you want to be notified of objects that are located. There are two major technologies used to identify buried items - target identification (Target ID) and sound-only systems. Target ID units display information on either Liquid Crystal Display (LCD) screens or use an arrow on a meter to point to the probable identification or a numeric readout, while the most basic sound system differentiates targets by generating variable tones based on the target’s size and metallurgical makeup. Some of the most common metal objects and valuable treasures have a similar magnetic footprint. With a little experience you’ll quickly learn to differentiate between all targets.
Gold and tab tops both emit similar magnetic signatures, and the subtle difference can be challenging. Accuracy in identifying all other metals and their actual depth is pretty amazing and quite simple. Target ID machines give you a visual and/or an audio signal that will indicate what range the target falls into, but you will not know for sure until you dig. With a little experience you will be able to more accurately discern what you’ve found before digging, but in the meantime it’s a good idea to take a look at all objects.
The depth at which an object can be located will depend on a number of factors, such as size of the object, soil iron/mineral content, the type of metal, technology used as well as the quality of metal detector. Generally speaking, most metal detectors can locate small objects such as coins up to six inches deep, while more powerful units can detect coins up to 12 inches deep and larger objects with a high magnetic signature down to five feet.
Some metals such as steel develop a halo effect after being in the ground a long time and that increases their magnetic signature, enabling them to be located at a greater depth. As a rule, given equal technology when comparing performance levels of two different detectors, the larger the search coil the greater depth of detection; however, smaller search coils can come in handy when searching around objects that have metal imbedded in them such as sidewalks or steel reinforced walls. Smaller search coils are also better at picking out the good objects where there’s a lot of metallic trash.
The type of technology as well as additional features increases a detector’s ability to probe the earth’s surface. However, other factors can have an affect on your ability to detect in certain areas. In an area where there is a high content of iron in the soil, detection is very challenging without a very sophisticated unit. Units with the automatic ground balance feature minimize this challenge and are affective about 95% of the time. If you live or intend on detecting in an area with a high mineral content, you’ll want to consider a unit that has a manual ground balance, but keep in mind that most detectors revert back to automatic settings for ground balance in the discrimination mode.
When selecting a detector, consider fit as well as features. The more sophisticated detectors today are designed to be ergonomically correct and are adjustable for varying heights.
Stabilizers
A stabilizer is simply a support that rests against your forearm and keeps the unit steady as you sweep it back and forth. Stabilizers add to the comfort of supporting the weight as well as preventing the unit from twisting. While stabilizers are optional on some units, all of the top manufacturers include them as standard equipment.
The Brains
The control box contains the electronic circuitry, signal display, controls, speaker, batteries and the microprocessor that analyzes the signals being received. The signal display can be either a meter or LCD screen.
Shaft
Shaft design on the more comfortable units is shaped in a flattened S, which positions the controls and LCD/Meters for easy viewing and adjustment access. Adjustable shafts are ideal for setting the reach to a comfortable distance based on your height and arm strength.
Search Coil
The search coil is the component that actually senses the metal. Search coils are also referred to as the "search head," "loop," or "antenna."
Audio Jack
Most systems also have a jack for connecting headphones. Using headphones will improve your metal detecting ability in two ways. Headphones eliminate background noise, which enables you to focus in on the various tones that indicate the presence of objects. Another important benefit of using headphones is extended battery life. Headphones require much less current to function, and this will increase the operating time significantly.
Target Selectivity
Discrimination is a feature available in most units, which enables users to filter out objects that are undesirable at the time. Since you may want to target a particular type of metal on one outing and another at a later date, the ability to adjust the level of discrimination is important.
Metal detectors can distinguish between different metal by capitalizing on a phenomenon known as phase shifting. Phase shifting is simply the difference in timing between the transmitter coil’s frequency and the frequency of the object being detected. Without going into a complicated, technical explanation, the amount of phase shift depends on the object’s magnetic inductance, which is keyed mostly on the object’s size, and on the object’s electrical resistance. Electrical resistance is a factor of both metallic composition, shape and its magnetic properties, such as iron. An object with high resistance changes its magnetic field quicker, which results in a smaller phase shift. Most metals have varying degrees of both inductance and resistance, which have to be evaluated. VLF detectors examine the amount of phase shift using a pair of electronic circuits called phase demodulators, then compares it to averages for a particular type of metal. By adjusting the discrimination feature you are essentially increasing or decreasing the threshold of detection to a desired level.
Notching is another important level of sophistication in discrimination technology. In a nutshell, a notch is a discrimination filter for a particular segment of phase shift. By setting up a notch you can filter out metals above and below a specific phase shift. Advanced detectors even allow you to program multiple notches. With this powerful tool you can set up specific notches to avoid being bothered with objects such as nails, tab tops or cast iron when searching an area for a specific metal such as silver.
Another feature to consider is the "No-Motion all metals mode." While operating in the discrimination mode, most detectors require motion over a metal object for a proper audio or visual response. With the "No-Motion all metals mode" option, as the name implies, proper detection is achieved with no motion and a constant signal will result from any metal target located anywhere under the coil. This mode is particularly effective for hunting relics in areas not prone to have a high level of trash metal. Using this option you’ll be able to detect objects much deeper because the unit is basically running full tilt, without any discrimination. The No-Motion mode is also preferred for pinpointing the exact location of an object that was detected in one of the other modes.
Another challenge for detectors is buried objects such as water pipes, phone and electric lines that can produce interference. It’s a good idea to know where utilities are buried in the area before you start digging with a lot of enthusiasm. The liability, not to mention the potential electric shock, is strictly yours. Always ask permission before detecting on private property and remember to close up the holes that you create so that a return visit will always be welcome.
Metal detecting can provide countless hours of amusement, not to mention the valuable items that you find. The places you can search are almost as unlimited as the variety of things you will find. From Civil War relics to coins and jewelry, every item located holds the promise of a great find waiting to be unearthed. Public places, such as parks, beaches and ball fields are often hit hard by other enthusiasts, but with every passing day more things are being lost. Somewhere there is a diamond ring, watch or old coin just waiting to be found by you, not to mention pirate treasure, outlaw loot and the treasure of the Sierra Madres.
There are numerous treasure sites on the Internet where you can research hundreds of instances where loot was reportedly buried and never recovered, not to mention the many jars of coins that were routinely buried during the depression and never recovered. All you need is a metal detector, earphones, a small digging tool, a little optimism, and the desire to look.
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